Modern high resolution photoresists require high photospeed for productivity, superior image integrity, and fully aqueous development and stripping. All aqueous processing is needed because the organic solvents that have been used in the past are being replaced because of increasingly demanding environmental restrictions.
In U.S. Pat. No. 3,779,778, novel photo-solubilizable compositions are disclosed that compromise 1) a water-insoluble compound containing one or more acid-degradable groups, and 2), a photoinitiator comprising a photolyzable acid progenitor. The useful water-insoluble compounds can contain one or more acid-degradable linkage which can be formed by the nucleophilic reaction of phenols, N-alkyl arylsulfonamides, or certain secondary amines with alkyl vinyl ethers, e.g. methyl vinyl ether, ethyl vinyl ether or a dihydropyran.
A dry film resist prepared according to this invention from the tetrahydropyranyl ether of a phenolic resin and a photoacid generator required a mixture of a flammable organic alcohol, potassium hydroxide and water to develop away the exposed areas.
European Patent Application 0 264 908 discloses resists comprised of at least one polymer, copolymer, or terpolymer having recurrent acid labile groups pendant to the polymer backbone wherein the improvement comprises: selecting a substituent side chain on said acid labile groups which is capable of forming secondary carbonium ion intermediates and having an available proton adjacent to the carbonium ion formed during cleavage. The autodecomposition temperature of the polymer comprising the photoresist is increased, to a temperature greater than about 160.degree. C., by selecting substituent side chains on the acid labile group which exhibit a less stable intermediate carbonium ion than the t-butyl ion.
In U.S. Pat. No. 4,491,628, resists sensitive to UV, electron beam and x-ray radiation with positive or negative tone upon proper choice of a developer are formulated from a polymer having recurrent pendant groups such as tert-butyl ester or tert-butyl carbonate groups that undergo efficient acidolysis with concomitant changes in polarity (solubility) together with a photoinitiator which generates acid upon radiolysis. A sensitizer component that alters wavelength sensitivity of the composition may also be added. The preferred acid labile pendant groups are tert-butyl esters of carboxylic acids and tert-butyl carbonates of phenols but, it is understood that a wide range of acid labile groups are operative in the invention. These include trityl, benzyl, benzhydryl modifications as well as others well known in the art.
A dry film resist prepared according to this invention from poly(tert-butyl acrylate) and a photoacid generator required heating of the exposed image at 100.degree. C. for up to 15 minutes in order to solubilize the polymer in fully aqueous base. The resulting developed image had ragged edges.
The production of printed circuits using a variety of resists is described in "Printed Circuits Handbook" edited by Clyde F. Coombs, Second Edition, published by McGraw-Hill, Inc., 1979. Chapter 6 of this publication describes the use of both negative-working and positive-working photoresists in the preparation of printed circuits. In particular, section 14 of this chapter discusses conventional positive-working liquid photoresists and section 16 discusses conventional coating methods for applying such photoresists to printed circuit substrates. Likewise, the production of printed circuits using photoresists is described in "Photoresist Materials and Processes" by W. S. DeForest published by McGraw-Hill Book Company, 1975. In particular, Chapter 5 of this publication discusses conventional positive resists and the processes of their use to manufacture printed circuits.
U.S. Pat. No. 3,469,982 discloses a process for forming a photoresist comprising (1) applying to a surface the surface of a solid unexposed photosensitive layer of thermoplastic photohardenable material, the other surface being adhered to a film support, then in either order (2) exposing the layer, and (3) stripping the support, and then (4) washing away the unexposed areas of the layer. The surface bearing the resist can be etched, plated, or treated in other ways. The photoactive materials disclosed in this case are photopolymerizable compounds.
Typical fully aqueous negative acting dry film photoresists used in the process described in this invention are described in Gilano, U.S. Pat. No. 3,887,450. They contain alkali soluble carboxylic acid containing polymers and photocrosslinkable acrylic acid monomers. Such compositions are developable in aqueous alkali and strippable in stronger aqueous alkali. However, they have marginal resistance, if any, to alkaline processes such as ammoniacal cupric chloride etchants or electroless copper plating baths which are typically used to form copper circuits. These negative acting coatings cannot be imagewise re-exposed with the same radiation used to produce the original image and developed to form a second image which could be used to modify the substrate a second time.
A very useful property of negative acting dry film photoresists is the capacity to cover or "tent" over holes in the substrate which need to be protected from the reagents which are used to modify the substrate. For example, if there are plated holes in the substrate that will ultimately provide electrical contact between circuit traces on both sides of the circuit panel, they must be protected during an etching operation that would be used to form the circuit traces. The tents often require a high level of exposure to crosslink them sufficiently so that they are not attacked by the alkaline development process; that level of exposure is often too high for optimum artwork reproduction.
U.S. Pat. No. 4,193,797 discloses a positive active dry film process for forming a photoresist comprising (1) applying to a surface the surface of a solid unexposed photosensitive layer of thermoplastic photosoluble or photodensitizable material, the other surface being releasedly adhered to a film support, then in either order (2) exposing the layer to actinic radiation, and (3) removing the film support, and then (4) washing away exposed areas of the layer to uncover areas of the underlying surface. The uncovered areas of the underlying surface can be etched, plated or otherwise modified. The photoactive materials disclosed in this invention include photodesensitizable bisdiazonium salt compositions, which required solvent development, and photosolubilizable o-quinone-diazides. These suffer from inherently high optical density in anything but thin coatings and slow photospeed. The thin coatings described in this invention (0.35 mils) would hardly be useful in typical dry film resist processes like plating and tenting.
The positive acting dry film resist compositions of the current invention are fully aqueous, have high photospeed, do not require the use of high temperature bakes after exposure to solubilize the exposed image, can tent over holes in the substrate, and give high resolution images with good line quality. Properly formulated, the resist image can withstand both alkaline and acid etching and plating processes typically used to modify substrates in the preparation of printed circuit boards. The resist image can then be blank exposed and stripped in aqueous alkali or re-exposed imagewise and developed so that the substrate can be modified a second time without the need to apply a second resist.
The resist compositions of the instant invention contain thermally stable but acid labile .alpha.-alkoxyalkyl esters of carboxylic acids, e.g., tetrahydropyranyl esters.
J. E. Kearns et al., J. Macromol. Sci.-Chem. A8(4), pp. 673-685 (1974) describe the preparation and deesterification of a number of polymers and copolymers of unsaturated tetrahydropyranyl esters. The utility of dihydropyran as a protecting group in the preparation of polymers containing other groups susceptible to normal hydrolysis conditions is ascribed to the mild conditions required to remove the tetrahydropyranyl group from these esters.
None of the prior art of which we are aware shows the use of the polymers of the present invention in dry film photoresist compositions. Moreover, none of the prior art of which we are aware discloses resist formulations usable in dry film photoresists containing a polymer bearing recurring pendant acid labile groups that possess the structures disclosed in the present invention.